Rapid growth in the use of organic light-emitting devices (OLEDs) is expected in the coming years due to their potential application in large screen flat panel displays. For full-color displays, efficient light-emitting diodes (LEDs) emitting three primary colors (i.e., blue, green and red) are required. However, obtaining strong red emission from conjugated polymers or small molecules is generally difficult because of the difficulty in obtaining sufficient conjugation length, and the gap law.
The use of triplet-based emitting centers in organic and polymer LEDs eliminates the 25 percent limit for maximum internal quantum efficiency, which is the expected singlet exciton fraction generated by electrical injection, and potentially allows for displays with 100 percent internal quantum efficiency. Strong back bonding with a metal center that exhibits a large spin orbit coupling constant facilitates intersystem crossing by breaking down the spin selection rules, leading to strong triplet state emission. This provides a possibility to design high efficiency OLED devices by using phosphorescent materials. Triplet-harvesting red and green LEDs based on platinum and iridium complexes have demonstrated very high external quantum efficiency. Europium complexes also show triplet emission and have also been used in red OLEDs. The characteristic of the lowest excited states (triplet states) of these heavy-metal complexes can be systematically varied from largely ligand-centered (LC) to metal-to-ligand-charge-transfer (MLCT) character. The triplet emission character depends upon the strength of the back bonding between the metal center and the ligand, and the relative energies of the π* (LC) transition versus the dπ* (MLCT) transition. The emission of europium complexes (sharp bands at around 615 nm) is completely inner shell electronic f to d transitions and is determined by the energetics of the central Eu3+ ion. The emission from platinum (II) porphyrins is ligand based, and iridium (III) complexes are largely ligand-based, although MLCT complexes have been reported for some iridium complexes as well. Luminescence of certain osmium (II) complexes being reported is from the MLCT state. Furthermore, these third row heavy-metal complexes tend to be thermally, chemically, and photochemically robust, which is favorable for device stability. Extremely long device lifetime has been reported for a triplet LED device using platinum octaethylporphorin (PtOEP) as LC emitting center with a 298° K triplet lifetime of about 50 μs. The long device lifetime is speculated to be an intrinsic property of electrophosphorescent LEDs, where radiative phosphors significantly shorten the lifetime of potentially reactive triplet states in the conductive host material. Due to strong back bonding from osmium to the ligands, the osmium complex triplet MLCT emission has a very short lifetime (from about 0.6 to about 1.8 μs).
Recently, red electrophosphorescence from osmium complexes has been reported. Jen et al., Applied Physics Letters, Vol. 80, No. 5, Feb. 4, 2002. Red electrophosphorescence from light-emitting diodes based on osmium complexes was achieved using in situ polymerized tetraphenyldiaminobiphenyl-containing polymers as the hole-transporting layer and osmium complexes doped blend of poly(N-vinylcarbazole) and 2-t-butylphenyl-1,3,4-oxadiazole as the emitting layer. The emission ranged from 620 to 650 mm. Because the emission originates from triplet metal-to-ligand-charge-transfer excited state, the emission, ranging from 620 to 650 nm, was tuned by changing the structures of the ligands. The peak external quantum efficiency and brightness achieved from the complexes were 0.82% and 970 cd/m2, respectively. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (x, y) for the best red emission from the complexes are (0.65, 0.33).
The reported osmium complexes were bis(4,4′-diphenyl-2,2′-bipyridyl) osmium (II) complexes. The complexes included either a phosphine or arsine bidentate ligand: 1,2-bis(diphenylarseno)ethane; cis-1,2-bis(diphenylarseno)ethylene; or cis-1,2-bis(diphenylphosphino)ethylene. The complexes further included two negatively charged counter ions: heptafluorobutylate (CF3CF2CF2CO2—) or p-toluenesulfonate (CH3C6H4SO3—).
Despite the advances in the development in osmium complexes for use in OLEDs, there exists a need for osmium complexes having greater brightness and higher quantum yields compared to existing osmium complexes. The present invention seeks to fulfill this need and provides further related advantages.